1. Field of the Invention
The invention relates to a friction material, which inhibits noise and vibrations during friction, and which improves moldability and reduces costs, and a production method of such a friction material.
2. Description of the Related Art
The brake pads, brake linings, clutch facings and other friction materials used in industrial machinery, railroad cars, cargo vehicles, automobiles and the like are required to have high reliability and increasingly high performance in order to ensure safety. More specifically, since friction materials are responsible for converting kinetic energy to heat through friction, they are required to have heat resistance against frictional heat generated during braking. At the same time, from the viewpoint of running stability, they are also required to demonstrate frictional properties that remain constant regardless of temperature or weather conditions, have superior wear resistance with little changes in properties over long period of time, and not generate noise such as squealing during braking or vehicle vibrations. In particular, noise and vibrations attributable to frictional vibration of the friction material are considered to be an important technical issue from the viewpoint of product value and quietness of vehicles.
In order to satisfy these requirements, friction materials are formed by combining several types of components. For example, fiber base materials for retaining the shape of the friction material, binders that bind components such as fiber base materials, and fillers for adjusting various properties of friction materials (such as adjustment and stabilization of wear resistance, heat resistance or friction coefficient) are used in combination. Friction materials are produced by curing a raw material mixture, which is obtained by mixing these components with a mixer, by hot-pressing followed by molding, grinding as necessary and sizing. Thermosetting resins as exemplified by phenol resins are frequently used as binders that compose friction materials due to their superior properties such as heat resistance, mechanical properties, low price and molding processability. Phenol resins are also subjected to modification for the purpose of improving the properties and performance mentioned above. However, adequate effects have yet to be obtained for heat resistance and frictional properties. Conventional phenol resins are decomposed by heat during friction or be melted by that heat, thereby having a detrimental effect on noise properties and vibration properties of friction materials. Although improvement of vibration absorption of friction materials and inhibition of the occurrence of squealing and other abnormal noise have been carried out by using modified phenol resin capable of lowering the hardness of friction materials, this conversely resulted in the problems of lowering the heat resistance of friction materials and reducing moldability during friction material production.
Therefore, a polyimide resin having superior heat resistance has been proposed for use as a binder (for example, Japanese Patent Publication No. 5-62916). This Japanese Patent Publication No. 5-62916 describes a polyimide-based friction material obtained by thermocompression molding a polyimide composition obtained by adding 5 to 30 parts by weight of a fluororesin powder and 1 to 30 parts by weight of a transition metal powder having unpaired electrons in the d orbital and/or an oxide powder of that transition metal to 100 parts by weight of an aromatic polyimide resin powder. In this Japanese Patent Publication No. 5-62916, after dry-mixing each raw material at the blending ratios indicated above with a Henschel mixer, the mixture is compression molded under conditions of 320 to 370° C. and 800 to 1500 kg/cm2 to produce a polyimide-based friction material. In the examples of this Japanese Patent Publication No. 5-62916, a polyimide friction material is actually produced that contains 71 to 89% by weight of polyimide resin.
However, this polyimide resin has the disadvantages of (1) being more expensive than phenol resin resulting in increased cost of the frictional material, and (2) requiring a large-scale and expensive production system as a result of having to be molded at high temperature and high pressure, thereby resulting in poor moldability and productivity. In addition, as in the friction material described in Japanese Patent Publication No. 5-62916, using a large amount of thermoplastic polyimide resins have the risk of (3) undergoing softening and melting due to frictional heat during braking, thereby causing phenomena such as fading and causing a decrease in the frictional properties of the friction material.